This invention relates to ring laser gyroscopes in general, and more particularly, to an improved pathlength controller for a ring laser gyroscope.
A ring laser gyro, as its name applies, is a gyroscope which utilizes a laser beam directed to travel in a closed path, e.g., a ring, to detect rotation about the axis of the path around which the laser beam is directed. Typical ring laser gyroscopes are disclosed in U.S. Pat. Nos. 3,373,650 and 3,467,472.
As disclosed in U.S. Pat. No. 4,160,184, the pathlength in a ring laser gyroscope must be closely controlled in order to maintain a constant scale factor and bias. This is usually accomplished both by the use of ultra-low expansion materials and by an active pathlength controller. Such a pathlength controller is disclosed in the aforementioned U.S. Pat. No. 4,160,184 in the form of a piezoelectric actuator.
In order for the ring laser gyroscope to operate with a very small drift and bias it is necessary to utilize an optical cavity with very small losses in the order of 300 ppm. Such a cavity lases with a quite low excitation current. It turns out that although this type of ring laser gyroscope operates with a single longitudinal mode in the absence of a rotational input when the pathlength is adjusted to the center of the lasing gain curve, the ring laser gyroscope will exhibit two or more longitudinal modes when the pathlength is adjusted away from the center of the lasing gain curve.
It is impossible to reduce the gain of such a high quality ring laser gyroscope by reducing the plasma excitation current, because the instrument is normally operated at a current level close to the critical dropout level, where the plasma extinguishes.
The servo loop controlling the pathlength utilizes a flexible mirror, attached to one corner of the ring laser gyroscopes, driven by a piezoelectric transducer. This transducer, when vibrated slightly at a known frequency, causes a frequency modulation of the ring laser gyroscope. If the laser is operated slightly off the peak of the laser gain curve, this frequency modulation also causes the ring laser gyroscope's laser intensity to be amplitude modulated. By demodulating, a direct current signal is obtained, which is used to control the transducer's position in order to restore the path length to the center of the laser gain curve.
This scheme works well for a ring laser gyroscope with relatively lossy cavity, that operates with a single longitudinal mode regardless of mistuning. However, when used with a ring laser gyroscope with a very low loss cavity that has more than one longitudinal mode, the path length controller will tune the ring laser gyroscope correctly only at some of the occasions when the ring laser gyroscope is turned on. In about one third of the occasions, when the ring laser gyroscope is turned on, the path length controller will tune such that two equal longitudinal modes are obtained. When this happens the ring laser gyroscope will not work at all. Because this is a stable and predictable operation of the servo, the only remedy available in prior art instruments was to turn the instrument off then on again.
More specifically, the prior art pathlength control system is shown on FIG. 1. It corresponds to the system described in conjunction with FIG. 6 of the aforementioned U.S. Pat. No. 4,160,184. In this system, an oscillator 14, typically operating at 2 KHz, provides one input to a high voltage amplifier 15 which drives a pathlength transducer, e.g., a piezoelectric transducer 13 having mounted to it a flexible mirror 12. The mirror 12 is shown forming, with a two fixed mirrors 10 and 11 respectively a triangular path for the laser beam. The basic purpose of the high voltage amplifier 15 is the supply of voltage to the transducer 13 which will maintain the proper pathlength. Impressed upon this voltage, which is a DC voltage, is a small AC voltage obtained because of the input from the oscillator 14. This causes the flexible mirror to vibrate slightly and to frequency modulate the laser beam 16. If the laser is tuned slightly off the center of the gain curve, an amplitude modulation also results. This amplitude modulation is detected by two detectors 17 and 18 which are disposed to detect the portions of the beam which are transmitted through the mirror 11 (as with any mirror, a small amount of light is transmitted and this transmitted light is what is detected by these detectors). Their outputs are summed and amplified in an operational amplifier 19, and then fed to a bandpass filter 20, the output of which is the input to a demodulator 21 having as a reference input the output of the oscillator 14. As a result, the output on line 22 from the demodulator will contain only the amplitude modulation and not the frequency of the oscillator, i.e., the 2 KHz signal. The demodulated voltage is then fed to an integrator 23 comprising an operational amplifier with an input resistor 25 and a capacitor 24 in its feedback path. This constitutes a closed loop servo system and the integrator 23 will integrate up and down until it reaches the desired operating point with the proper pathlength.
If one opened the servo loop and looked at the output of the demodulator 22 it would appear as shown on FIG. 2. The curve shown on FIG. 2 result from tuning through the single longitudinal modes and from tuning through the modes where the cavity has two equal longitudinal modes. The zero cross-overs marked with circles 40 represent tuning to a single longitudinal mode and the zero cross-overs 41 indicated by the x's represent tuning at points where there are two equal longitudinal modes. Note that the amplitude associated with the single longitudinal mode is considerably greater than the associated with the two longitudinal modes. Proper operation occurs only if the system is tuned to one of the zero cross-overs 40 corresponding to the single longitudinal mode. Thus, as noted above it can be seen that it is possible for the servo loop to null at either one of the points 40 or 41 in this circuit of the prior art.
Quite clearly a mode of operation in which one must turn the gyro on and off until operation in the proper mode occurs is not desirable, particularly if the ring laser gyroscope is to be used in a navigational system. It is thus apparent that there is a need for a simple and effective manner of improving the servo loop so that it will always tune the ring laser gyroscope to the center of the gain curve, i.e., so that it will always tune to one of the proper zero crossovers 40 of FIG. 2.